In recent years, liquid crystal devices (LCDs) are currently most commonly used as a flat panel display (FPD) due to the advantage of light weight and low power consumption.
However, the liquid crystal devices are not a self light emitting element but a light receiving element and have technical restrictions in brightness, contrast, viewing angles, large size, etc. Thus, recently, the efforts to develop new flat panel displays for overcoming such disadvantages have been actively pursued.
An organic light emitting diode, one of the new flat panel displays, is superior to a liquid crystal display in viewing angles, contrast, etc. because it is a self light emitting type, and can be made lightweight and thin, and is advantageous from a power consumption point of view because it requires no backlight.
Additionally, the organic light emitting diode has an advantage that it is strong to an external shock, provides a wide range of temperature because it is capable of direct current low voltage driving, has a fast response speed, and is made entirely in a solid phase. Furthermore, it has a cheap manufacturing cost.
In a manufacturing process of the organic light emitting diode device, all that is needed is deposition and encapsulation equipment unlike a liquid crystal device or PDP (plasma display panel), thus the process is very simple.
If the organic light emitting diode device is driven in an active matrix type having thin film transistors, which are switching devices for each pixel, it shows the same luminance even if a low current is applied. This enables low power consumption, high definition, and large size.
FIG. 1 is a view showing a basic structure of a general active matrix type organic light emitting diode device (AMOLED). In FIG. 1, the general organic light emitting diode display panel comprises gate lines GL1˜GLm and data lines DL1˜DLn arranged to cross each other on a glass substrate with pixel portions 30 formed respectively in rectangular regions of a matrix pattern defined by the gate lines GL1˜GLm and the data lines DL1˜DLn crossing each other.
The pixel portions 30 are driven in units of gate lines GL1˜GLm by a scanning signal applied via the gate lines GL1˜GLm, and generates light corresponding to the intensity of image signals applied via the data lines DL1˜DLn.
Therefore, in the organic light emitting diode display panel, a scanning line driving circuit 10 for applying scanning signals to the gate lines GL1˜GLm and a data driving circuit for supplying image signals to the data lines DL1˜DLn are manufactured on a single crystal silicon substrate, and attached on a glass substrate of the organic light emitting diode display panel in the same method as a taper carrier package (TCP).
In the image display portion, a plurality of gate lines GL1˜GLm arranged in a transverse direction at regular intervals and a plurality of data lines DL1˜DLn arranged in a column direction at regular intervals cross each other. In the regions defined by the gate lines GL1˜GLm and the data lines DL1˜DLn crossing each other, pixels 100 electrically connected to the gate lines GL1˜GLm and the data lines DL1˜DLn are respectively provided.
The pixels 100 are driven in units of gate lines GL1˜GLm by a scanning signal applied via the gate lines GL1˜GLm, and generates light corresponding to the intensity of image signals applied via the data lines DL1˜DLn.
FIG. 2 is a circuit diagram showing a unit pixel of a general active matrix type organic light emitting diode device. In FIG. 2, a gate line GL is formed in a first direction, and a data line DL and a power supply line VDD formed at a given interval in a second direction crossing the first direction, thereby forming one pixel region.
A switching thin film transistor TR2, an addressing element, is connected to the region where the gate line GL and the data line DL intersect. A storage capacitor (hereinafter, referred to as Cst) is connected to the switching thin film transistor TR2 and the power supply line VDD. A driving thin film transistor TR1, a current source element, is connected to the storage capacitor Cst and the power supply line VDD, and an electroluminescent diode EL is connected to the driving thin film transistor TR1.
The switching thin film transistor TR2 includes a source electrode S1 connected to the gate line GL and supplying a data signal and a drain electrode D1 connected to a gate electrode G2 of the driving thin film transistor TR1, and which switches the electroluminescent diode EL.
The driving thin film transistor TR1 includes a gate electrode G2 connected to the drain electrode D1 of the switching thin film transistor TR2, a drain electrode connected to an anode electrode of the electroluminescent diode EL and a source electrode S2 connected to the power line VDD, and serves as a driving device of the electroluminescence diode.
In the storage capacitor Cst, an electrode at one side is commonly connected to the drain electrode D1 of the switching thin film transistor TR2 and the gate electrode of the driving thin film transistor TR1, and an electrode at the other side is connected to the source electrode S2 and of the driving thin film transistor and the power line VDD.
The electroluminescence diode EL includes an anode electrode connected to the drain electrode D2 of the driving thin film transistor TR1, a cathode electrode connected to the ground line GND and an organic light emitting layer formed between the cathode electrode and the anode electrode. The organic light emitting layer is comprised of a hole carrier layer, a light emitting layer and an electron carrier layer.
The thus-constructed general organic light emitting diode device (AMOLED) supplies currents through the thin film transistors. Because conventional amorphous silicon thin film transistors are low in carrier mobility, polysilicon thin film transistors with improved carrier mobility have been employed in recent years.
In order to show a minute color change, a good gray scale capability is a must-have function in displays.
The aforementioned organic light emitting diode device displays images by controlling the amount of current flowing in the electroluminescence diode. The organic light emitting diode device displays gray scales by differentiating the amount of light emission of the organic light emitting diode device by controlling the amount of current flowing in the thin film transistors for supplying currents to the organic light emitting diode device in an active driving method.
However, according to a driving circuit and driving method of an organic electroluminescence display device according to the conventional art, the current of the organic light emitting diode is determined according to a gate voltage VIN of a driving polycrystalline silicon thin film transistor TR1.
The driving polycrystalline silicon thin film transistor TR1 operates in a saturation region, thus a flowing current is expressed by the following formula (1):IDS=W/L μp COX (VDD−VIN+VTH)2  (1)
wherein W denotes a channel width of the driving thin film transistor, L denotes a channel length, μp denotes a charge transfer rate, VDD denotes a power supply line, VIN denotes a gate voltage, and VTH denotes a threshold voltage.
If the threshold voltage of the driving polycrystalline silicon thin film transistor TR1 between panels is changed, the current of the driving polycrystalline silicon thin film transistor TR1 and the current of the organic light emitting diode are also changed, thereby making the luminance between panels non-uniform.